Wireless communication systems have, in recent years, seen a tremendous growth surge. Advances in signal processing, driven by the demand for high speed data as well as improvements in spectral efficiency (such as for voice users), have made the balance between radio capacity and available user channels more complicated. For example, recent standards and equipment modification proposals relating to the use of high speed data (HSD) will further contribute to the problem. Such units will be capable of receive and/or transmit operations on multiple channels such as supplemental channels, a fundamental channels and dedicated control channels simultaneously.
In cellular systems, code channel availability as well as overall system capacity can be increased when cells are made ever smaller (more cells in a given area), however, various issues, including cost, efficiency and interference from transmissions in other cells, prevent such action from being a total solution.
In code division multiple access (CDMA) systems, the number of users that can be accommodated is a function of the available code channels. Typically, CDMA systems use Walsh codes, a set of orthogonal codes, where the number of codes available equals the chip rate divided by the data rate. Channels encoded with Walsh codes are called “Walsh channels” or “code channels.” It is desirable to use orthogonal codes in the forward link (FL) since much of the inter-channel interference cancels when orthogonal codes are used. As should be apparent, the FL comprises communication from the base transceiver station (BTS) to the mobile station (MS).
Typically, spread-spectrum communications systems such as CDMA, employ pseudo-random noise (PN) codes for spreading the communication signal to the desired bandwidth. As is well known in the art, a PN code is comprised of chips where a chip may be equated to a unit of time duration. A PN sequence of chips may be used in CDMA as a scrambling code. Following data modulation via a phase shift keyed output signal of a given modulation order, prior art CDMA, PCS and cellular communication systems, added Walsh codes and PN spreading combined in a well known manner. Known systems have used binary phase shift keyed (BPSK) and quadrature phase shift keyed (QPSK) modulation orders.
In general there are N orthogonal codes for a code of length N bits (chips). This also applies to Walsh codes, wherein there are N length N orthogonal Walsh codes. It may be noted that the PN chip rate is the same as the Walsh chip rate. In order to have consistent numerology the PN chip rate must equal the modulation symbol rate times the Walsh code length in chips.
In the design of some prior art systems using QPSK, a data rate into the encoder of 9,600 symbols per second, and a PN chip rate of 1.2288 Mcps (mega chips per second) allowed for 128 code channels to be transmitted simultaneously from a BTS antenna when the radio environment supports that many users (or user channels).
PCS and cellular communications systems often encounter various types of radio environments. For example, the radio signal may encounter various degrees of fading due to multipath and mobile velocities. Other factors such as shadowing may also cause a reduction of signal strength between transmitter and receiver. These same obstacles may also cause signal reflection which results in multipath signals that tend to confuse the receiver in determining what signal to detect. Some of these problems may be overcome by increasing the power of the transmitted signal. In view of the above, the radio environment may be such that the BTS (forward link) runs out of transmitter power before the number of code channels (Walsh codes) available are exhausted. It is generally deemed desirable for the radio environment to limit the system capacity rather than the number of available code channels. However, there may be situations in a given system when the available Walsh channels are exhausted before the BTS power limit is reached. In this case, the capacity of the system is artificially limited by the Walsh code channels rather than the radio environment.
BPSK (modulation order of 2) systems are simpler to implement than are QPSK systems since the signal processing complexity is greater for the latter. While an 8 or other higher order system might immediately come to ones mind as a way to solve the problem of having an adequate number of Walsh codes, other considerations must be addressed. If the transmissions employ higher order modularity (M>4), then all users must purchase new equipment to use the system. Further, the transmissions must remain orthogonal in order for the system to be usable and/or practical, numerology must be accommodated and so must FEC coding. Just because BPSK and QPSK systems proved to be capable of providing orthogonality, does not mean that higher order modulation schemes are also orthogonal. With proper design, the result of which will be revealed below, these considerations can be satisfied. Therefore, it would be desirable to use a higher modulation order system when both the radio environment and the mobile capability can support a higher modulation order.